1. Field of the Invention
This invention relates to a thermal imaging system of the kind in which a scene is imaged by scanning.
2. Discussion of Prior Art
Thermal imaging systems are known in the prior art. In principle, infrared radiation emitted by a warm object is directed onto a photoconductive detector (see, for example, Thermal Imaging Systems by J. M. Lloyd, Plenum Press, 1975) and the thermal image is reconstructed from the electrical response. Coverage of a large scene area is generally achieved in one of two ways: imagers either sample simultaneously different sections of a scene or image each component of a scene sequentially. In the former case an array of detectors are used in a "staring" mode and in the latter a mechanical scanning mechanism directs radiation from each pixel of a scene onto a small detector area. A staring array imaging system has the advantage that it lends itself to compactness of design but suffers from many other problems. Detecting elements rarely have uniform detectivities and responsivities, 1/f noise is relatively important and effective cold shielding poses further problems. Scanning thermal imagers, although more cumbersome, offer considerable advantage in requiring simpler and cheaper detector arrays. A third option is to use a combination of both methods, namely, mechanical scanning of a large scene area onto a small array of detectors. Individual scene pixels may be scanned sequentially over a number of detectors, the signals from which are time delayed and integrated in the thermal image resulting in either an improved sensitivity of image or acceptable sensitivity being achieved with faster scan speeds. Furthermore, several pixels may be scanned simultaneously by the use of multiple detectors.
There are considerable differences to be borne in mind when imaging thermal wavelengths as opposed to optical photons. First, optical images are produced primarily by reflection and reflectivity differences, and in this spectral region terrestrial materials tend to have good reflectivities. Thermal images arise from self-emission and emissivity differences and against a terrestrial background radiation contrasts are small and so object resolution is coarse. More effort has to be directed towards improving the contrast within a thermal image. Secondly, although it is electromagnetic radiation which is detected, a thermal image is generally described in terms of temperature. All contributions to the measured irradiance at any point in a scene can conveniently be represented by an effective temperature at that point. This temperature is that at which an ideal black body would emit radiation with the measured intensity.
A major advantage of scanning thermal imagers is that the detector signal can be ac coupled to the signal processing system. Commonly a scene is dissected by raster scanning into a series of parallel lines covering its area. Insertion of a high pass circuit between detector and amplifier eliminates the dc component of a signal and transmits only variations as the scan progresses along a line. This has the advantages of enhancing the contrast, eliminating any variability in the dc offset voltage of different detectors and reducing the effects of detector 1/f noise. Image defects arise however: a step function signal from the detector will be distorted as capacitance within the high pass circuit discharges and the output signal will suffer from droop which in turn causes undershoot as the input step function returns to zero. In addition, the response of a high pass circuit is somewhat dependent on its past history; in a multielement detector it is highly unlikely that all such circuits will have received the same average signal and so channel responses will not be uniform. Furthermore elimination of the dc component also removes the absolute temperature of the scene.
It is advantageous to restore some absolute temperature reference to the image. This can be important if, for example, the horizon is to be seen and for radiometric monitoring of, for example, manufacturing processes. To achieve this and also to counter the image defects of droop, undershoot and channel nonuniformity a prior art technique gives a reference signal to the detector at intervals during the scan. The reference source is typically a convenient passive scene such as the interior of the imager housing although an active source such as a heated strip can also be used. The image is then displayed with the high frequency pixel to pixel variations within the scene superimposed on the dc level provided by the reference temperature.
This method of artificial dc restoration is not ideal if the reference temperature is not close to the scene average. An intermediate frequency variation in temperature will occur as the detector switches between scene and reference source and this will not be immune to the effects of droop and undershoot. Furthermore detector responsivity can vary widely with illumination intensity and so unifying the response of a multidetector system at the reference temperature will not guarantee a consistent response at the scene temperature. Unfortunately this problem is not easily overcome. The situations in which thermal imaging systems are employed are many and varied. Portability is frequently essential and it is not practical to provide as many different temperature reference sources as there are scenes likely to be imaged. An active source requires heating apparatus or more bulky cooling apparatus and the time taken to reach desired temperatures is often prohibitive.
An example of a prior art multidetector scanning thermal imager which primarily addresses the problem of non-uniformity of detector response is disclosed in U.S. Pat. No. 4,948,964. A chopping mirror is arranged such that during the reset portion of the scan, two reference illuminations of differing intensities are incident on the detector array. For this intensity (temperature) change, the gain of each detector is adjusted to ensure that uniform voltage responses are developed. The thermal reference source used is a thermoelectric cooler which, if possible, is adjusted to the approximate average temperature of the scene in order that the gain normalisation is performed within the dynamic range of the imager. This average level provided by the cooler is also used as the clamping level in dc restoration.
Alternative prior art radiation sources, although not disclosed in the context of imaging processes, are disclosed in U.S. Pat. No. 4,561,786 to Anderson and by Bolgov et al. in "IR Sources with Barrier-Free Injection Mechanism", Avtometriya, No. 4, pp 85-88, (1989). The former document discloses use of a light emitting diode (LED) to provide a reference signal which is of equivalent brightness to target (scene) emission at a particular wavelength in constructing a radiometer which is relatively insensitive to surface characteristics of the target being measured. Bolgov discloses a crystal plate of intrinsic InSb coated and etched such that one face of the plate has a high carrier recombination rate (s.sub.max) and the opposite face has a low recombination rate (s.sub.min). The Hall effect is then used to concentrate carriers at one or other of the faces. When carriers are concentrated at the s.sub.min face, the lower recombination rate results in an above-equilibrium distribution of carriers at this surface and luminescent emission is observed. Conversely, concentration of carriers at the s.sub.max face results in the crystal becoming depleted of charge carriers and below-equilibrium radiation emission is observed from the s.sub.min face.